LOW PERMEABILITY HIGH PRESSURE COMPRESSOR ABRADABLE SEAL FOR BARE Ni AIRFOILS HAVING CONTINUOUS METAL MATRIX

ABSTRACT

An air seal in a gas turbine engine comprising a substrate. A bond coating layer is adhered to the substrate. An abradable layer is adhered to the bond coating layer. The abradable layer comprises a metal matrix discontinuously filled with a soft ceramic material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/689,492 which claims the benefit of U.S. Patent ApplicationSer. No. 61/983,590, filed Apr. 24, 2014.

BACKGROUND

The disclosure relates to an air seal for a gas turbine engine.

In compressor and turbine sections of a gas turbine engine, air sealsare used to seal the interface between rotating structure, such as a hubor a blade, and fixed structure, such as a housing or a stator. Forexample, typically, circumferentially arranged blade seal segments arefastened to a housing, for example, to provide the seal.

Relatively rotating components of a gas turbine engine are not perfectlycylindrical or coaxial with one another during engine operation. As aresult, the relatively rotating components may occasionally rub againstone another. To this end, an abradable material typically is adhered tothe blade seal segments or full rings and/or the rotating component.

Abradable seals in the compressor section of gas turbine engines includecharacteristics such as, good abradability, spall resistance, anderosion resistance. Abradable seals are required to exhibit a smoothsurface, low gas permeability, and environmental durability. The seal isa sacrificial element in order to minimize blade wear, so it isabradable. The seal must also minimize gas flow leakage through theseal, so a low gas permeability is desirable.

Abradable coatings for the seals are always a compromise betweenabradability and erosion resistance. In order to maintain blade tipclearances over time, the seal material needs to be tough and resistantto erosion. Conventional seal materials tend to be soft and weak inorder to have good abradability. However, current understanding of thewear mechanisms involved with high pressure compressor (HPC) abradablematerials, while rubbed with bare Nickel (Ni) alloy blades, is that weartakes place by adhesive wear mechanisms, plastic deformation andfracture on a scale far smaller than the size of coating constituentparticles. This becomes apparent when one considers that the wear perblade passage is on the order of 1E-6 inches.

The new understanding of the scale of wear particle removal providesinsight that leads to an improved coating structure that minimizesvolume fraction of the strong metal constituents while assembling themin a structure that maximizes their contribution to toughness andstrength.

SUMMARY

In accordance with the present disclosure, there is provided a sealcomprising an abradable layer, the abradable layer comprising a metalmatrix discontinuously filled with soft ceramic material. The softceramic material comprises soft ceramic particles clad with a metallicalloy.

In another alternative embodiment an air seal in a gas turbine enginecomprises a substrate. A bond coating layer is adhered to the substrate.An abradable layer is adhered to the bond coating layer. The abradablelayer comprises a metal matrix discontinuously filled with hexagonalboron nitride clad with a metallic alloy.

In another and alternative embodiment, the substrate is metallic.

In another and alternative embodiment, the metal matrix isdiscontinuously filled with a hexagonal boron nitride or a hexagonalboron nitride agglomerate.

In another and alternative embodiment, the metal matrix isdiscontinuously filled with a soft phase material. The abradable layerincludes a metal fraction of from about 10 V% to about 36 V%.

Further in accordance with the present disclosure, there is provided agas turbine engine comprising a first structure and a second structurerotating relative to the first structure, wherein one of the firststructure and second structure comprises a substrate. A bond coatinglayer is adhered to the substrate. An abradable layer is adhered to thebond coating layer. The abradable layer comprises a metal matrixdiscontinuously filled with hexagonal boron nitride clad with a metallicalloy.

In another and alternative embodiment, the substrate is an outer case,and the other rotating structure is a blade tip, wherein the blade tipis arranged adjacent the outer case without any intervening, separableseal structure.

Further in accordance with the present disclosure, there is provided amethod of manufacturing a gas turbine engine air seal comprisingdepositing a bond coating onto a substrate. The method includesdepositing an abradable coating onto the bond coating. The methodincludes cladding hexagonal boron nitride particles with a metallicalloy and consolidating the clad boron nitride particles.

In another and alternative embodiment, the method further comprisesplasma spraying the abradable coating onto the bond coating.

In another and alternative embodiment, the method includes depositingthe abradable coating onto a substrate and depositing includes at leastone of hot pressing the abradable coating directly onto the substrate,as a pressed and sintered biscuit that is brazed on, glued, mechanicallyattached, attached by hot isostatic pressing, and sprayed directly ontothe substrate.

In another alternative embodiment, the abradable coating furthercomprises metal clad hBN particles with additional metal matrixparticles, metal particles of a different composition to the cladding,fugitive pore formers, and additional soft phase material in thecomposite powder. The fugitive pore formers may comprise at least one ofa polyester particle and a Lucite particle. The additional soft phasematerial may comprise a bentonite agglomerated hBN, molybdenumdisilicide, or a MAX phase ternary carbide or nitride.

In another alternative embodiment, the method includes adjusting theabradable coating properties during manufacture to target the propertiesrequired for a predetermined gas turbine engine section environment.This may be done in by adjusting the ratio of constituent materials(powders.) In an exemplary embodiment, the adjusting step furthercomprises adjusting a ratio of the clad hBN particles to at least one ofthe additional metal matrix particles, the fugitive pore formers, andthe additional soft phase material in a composite powder.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a portion of a gas turbine engineincorporating an air seal.

FIG. 2 shows a schematic view of an air seal.

FIG. 3 shows a cross sectional view of a coating powder before beingapplied.

FIG. 4 shows a cross sectional view of a coating on a substrate.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a portion of a gas turbine engine 10, for example, a highpressure compressor section. The engine 10 has blades 15 that areattached to a hub 20 that rotate about an axis 30. Stationary vanes 35extend from an outer case or housing 40, which may be constructed from anickel alloy, and are axially interspersed between stages of the turbineblades 15, which may be constructed from titanium in one example. Afirst gap 45 exists between the blades 15 and the outer case 40, and asecond gap 50 exists between the vanes 35 and the hub 20.

Air seals 60 (FIG. 2) are positioned in at least one of the first andsecond gaps 45, 50. Further, the air seals 60 may be positioned on: (a)the outer edge of the blades 15; (b) the inner edge of the vanes 35; (c)an outer surface of the hub 30 opposite the vanes 35; and/or (d) asshown in FIG. 2, on the inner surface of outer case 40 opposite theblades 15. It is desirable that the gaps 45, 50 be minimized andinteraction between the blades 15, vanes 35 and seals 60 occur tominimize air flow around blade tips or vane tips.

In one example shown in FIG. 2, the air seal is integral with andsupported by a substrate, in the example, the outer case 40. That is,the air seal 60 is deposited directly onto the outer case 40 without anyintervening, separately supported seal structure, such as a typicalblade outer air seal. The tip of the blade 5 is arranged in close,proximity to the air seal 60. It should be recognized that the sealprovided herein may be used in any of a compressor, a fan or a turbinesection and that the seal may be provided on rotating or non-rotatingstructure. The seal can also be for a turbine pump in a gas pipeline, awater or oil seal in a pump or other application.

The air seal 60 includes a bond coat 65 deposited onto the outer case40. In an exemplary embodiment, the bond coat 65 may be a thermallysprayed bond coat. In another example, the bond coat 65 may comprise analloy, such as PWA1365 MCrAlY composition applied by air plasma spray.In another exemplary embodiment, the bond coat 65 can be optional, if itis used, the bond coat 65 can be thermally sprayed, a braze material ora polymer adhesive. A composite topcoat 70 acts as an abradable layerthat is deposited on the bond coat 65 opposite the outer case 40. In anexemplary embodiment, the metallic bond coat 65 may be replaced by anadhesive layer. The adhesive may be polyurethane in the front stages ofthe compressor or in the fan where ambient temperature is sufficientlylow (e.g., less than about 300 degrees Fahrenheit.

Referring also to FIGS. 3 and 4, the composite abradable coating 70consists of a material that is a single distribution of a hexagonalboron nitride (“hBN”) 100 or soft ceramic material or other soft phaseclad with a metallic-based alloy cladding 102 (such as a Ni based alloy,though others such as cobalt, copper and aluminum are also contemplatedherein). Feed stock used to provide the air seal 60 abradable coating 70is made of composite powder particles of Ni alloy and hBN in which themetal is plated onto the hBN in a chemical cladding process, the metalclad hBN particles are used at a variable ratio with additional metalparticles, fugitive pore formers, such as polyester or Lucite particlesor additional soft phase material (Such as bentonite agglomerated hBN)in the composite powder to adjust and target the coating propertiesduring manufacture. In an exemplary embodiment, the additional metalparticles may be the same composition as the plating or different. Theadditional particles can be alloying elements such as Al, Cr, Si, Bwhich may serve as a processing aid or modify the matrix alloy toprovide some desired property such as oxidation resistance. It may bedesirable to add Cr and/or Al and the like, as separate particles. Thecomposition of these particles may advantageously combine with thematrix metal to improve oxidation resistance or other property (bydiffusion during heat treatment or in service. Other compounds such as arelatively soft (3 or less or preferably less than 2 on the Mohshardness scale) ceramic like bentonite clay (e.g., a Montmorillonite)may be substituted for the hBN.

The matrix 102 of Ni based alloy may be coated upon the hBN 100 beforethermal spraying. In an exemplary embodiment, the metal cladding mayalso be produced as discrete elemental layers in order to facilitatemanufacturing as it is difficult to co-deposit multiple elements as analloy in the cladding process.

The volume fraction of hBN in the composite coating 104 is about 50-80%.The target metal content of the coating may be around 50% by volume orless. In one example, a volume fraction of hBN in the range of 75-80% isused. The target metal fraction can be on the order of 10-36% by volume.Some porosity, 0.5 to 15 volume % is normal in thermal spray coatingsdepending on the process and material. A low volume fraction of fugitivemay be desirable to further reduce density and rub forces withoutsubstantially affecting roughness and gas permeability (e.g., less thanabout 25 volume %).

An additional volume fraction may be porosity which is inherent to thethermal spray process or intentionally induced with spray parameterselection or the addition of a fugitive material. Example fugitivematerials are polyester and Lucite powders. The low volume fraction ofmetal in combination with the hBN limits the ductility of a surfacelayer that forms by mechanical alloying due to plastic deformation as itis rubbed by an airfoil tip (or other rotating element) which results ingood abradability. Low volume fraction of metal and poor bonding withthe hBN also produces a low modulus composite that is somewhat flexibleand compliant to part deformation and thermal expansion contributions tostress. The low modulus keeps stresses low.

It should be noted that the ductile matrix phase provides toughness,erosion resistance, spallation and cracking resistance while theselection of matrix and filler combine to provide specific properties ofthe mechanically alloyed surface layer in order to promote abradability.The hBN is particularly well suited to forming the low ductility surfacelayer because hBN does not bond well to the metal and when mixed intothe metal weakens it, lowers the ductility and promotes the removal ofwear particles from the surface.

The metal and hBN composite coating bonds with the bond coat 65 throughmechanical interlocking with the rough surface of the thermally sprayedbond coat 65, which provides a durable, low stress abradable layer thatwill remain bonded to the bond coat 65 during engine service includingrub events. The topcoat abradable layer 70 can be deposited through avariety of methods. In an exemplary embodiment, the abradable layercould be hot pressed directly onto the part, as a biscuit that is brazedon, glued or mechanically attached, attached by hot isostatic pressing,pressed and sintered, as well as sprayed directly onto the substrate orbond coat. The powders are deposited by a known thermal spray process,such as high velocity oxygen fuel spraying (HVOF), and air plasma spray(APS) or cold spray.

One or more embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, thepresent disclosure seeks to provide a strong continuous network of metalmatrix that is discontinuously filled with soft phase like hBN or hBNagglomerates. This is accomplished in an efficient manner by metalcladding hBN or hBN agglomerates and depositing them by plasma spraymethods. The metal cladding results in efficient spray deposition andwell distributed soft phase that is surrounded by an interconnectedmetal matrix. The interconnectivity of the matrix provides high strengthand toughness for the relatively low volume fraction or metal. Targetmetal fraction is on the order of 10-36 V%. The low volume fraction ofmetal in combination with the hBN limits ductility of the smeared(mechanically alloyed) layer resulting in good abradability. The presentcoating structure and composition results in improved toughness, erosionresistance for a given metal content while maintaining abradability. Thecomposition and structure provides low roughness and low gaspermeability due to near fully dense coating structure. Roughness can bereduced due to the well distributed phases and low porosity comparedwith conventional coating composite structures. Accordingly, otherembodiments are within the scope of the following claims.

1-7. (canceled)
 8. A gas turbine engine comprising: a first structure; asecond structure rotating relative to the first structure, wherein oneof the first structure and second structure comprises a substrate; andan abradable layer adhered to the substrate, the abradable layercomprising a metal matrix discontinuously filled with soft ceramicmaterial clad with a metal, wherein the metal cladding comprisesmultiple discrete elemental layers.
 9. The gas turbine engine of claim8, wherein the substrate is an outer case, and the other rotatingstructure is a blade tip, wherein the blade tip is arranged adjacent theouter case without any intervening, separable seal structure.
 10. Thegas turbine engine of claim 9, wherein the metal matrix isdiscontinuously filled with a hexagonal boron nitride agglomerate. 11.The gas turbine engine of claim 8, wherein said metal matrix isdiscontinuously filled with a soft phase material.
 12. The gas turbineengine of claim 8, further comprising: a bond coating layer adhered tothe substrate; and said abradable layer adhered to said bond coatinglayer.
 13. A method of manufacturing a gas turbine engine air sealcomprising: depositing an abradable coating onto a substrate, includingcladding soft ceramic material particles with a metallic alloy, whereinthe metal cladding comprises multiple discrete elemental layers andconsolidating the clad boron nitride particles.
 14. The method ofmanufacturing a gas turbine engine air seal of claim 13 furthercomprising: plasma spraying the abradable coating onto the substrate.15. The method of manufacturing a gas turbine engine air seal of claim13 wherein said depositing said abradable coating onto a substrateincludes at least one of hot pressing said abradable coating directlyonto the substrate, as a pressed and sintered biscuit that is brazed on,glued, mechanically attached, attached by hot isostatic pressing, andsprayed directly onto the substrate.
 16. The method of manufacturing agas turbine engine air seal of claim 13, wherein said abradable coatingfurther comprises at least one of additional metal matrix particles,fugitive pore formers, and additional soft phase material in a compositepowder.
 17. The method of manufacturing a gas turbine engine air seal ofclaim 16 further comprising: adjusting said abradable coating propertiesduring manufacture to target the properties required for a predeterminedgas turbine engine section environment.
 18. The method of manufacturinga gas turbine engine air seal of claim 17 wherein adjusting furthercomprises adjusting a ratio of said clad hBN particles to at least oneof said additional metal matrix particles, said fugitive pore formers,and said additional soft phase material in a composite powder.
 19. Themethod of manufacturing a gas turbine engine air seal of claim 16wherein said fugitive pore formers comprise at least one of a polyesterparticle and a Lucite particle.
 20. The method of manufacturing a gasturbine engine air seal of claim 16 wherein said additional soft phasematerial comprises a bentonite agglomerated hBN.